1. Field of the Invention
This invention relates to a light source device for measuring shape, and in particular a light source device which measures the shape of a surface by irradiating the surface with a laser beam like slit shape and receiving the light reflected from the surface.
2. Description of the Related Art
A device is known in the art which measures the shape of surface undulations and other features without contact by a triangular quantization method (Japanese Patent Application Publication No. 50-36374, Japanese Patent Application Laid-Open No. 56-138204, Japanese Patent Application Laid-Open No. 57-22508 and Japanese Patent Application Laid-Open No. 58-52508).
In this shape measuring device, a slit type laser beam (referred to hereinafter as a slit beam) irradiates a surface to be measured, and light reflected from the surface irradiates on a sensor disposed at a predetermined angle with respect to the optical axis of the irradiating beam. Surface features are then measured by measuring the amount of reflected light reaching the sensor from the surface. More specifically, as shown in FIG. 3, the shape measuring device 12 comprises a semiconductor laser 50, a light source device 10 provided with a collimator lens 52 and a dispersing lens 54, a light receiving device provided with a light receiving lens 56 and photodetector 58, and a operating circuit 62 connected to the photodetector 58. The aforesaid dispersing lens 54 may consist of a rod lens or a cylindrical lens. The photodetector 58 may consist of CCD sensor or PSD element.
In the shape measuring device 12, light emitted by the semiconductor laser 50 is made to converge on a surface 60A to be measured by means of the collimator lens 52. The shape of the laser beam emerging from the collimator lens 52 is modified to that of a slit beam by the diffusing lens 54 before irradiating the surface 60A, The bright line is reflected at the surface 60A, collected by the receiving lens 56, and made to impinge on the photodetector 58. An image of the bright line in the surface 60A (referred to hereinafter as a slit image) is thereby formed on the photodetector 58, which outputs an electrical signal depending on the position of the slit image to the operating circuit 62.
As shown in FIG. 5, the aforesaid light source device 10 is provided with a lens barrel 30, the collimator lens 52 being fixed to the inside of this lens barrel 30. The diffusing lens 54 is fixed to the lens barrel 30 on the right-hand side of FIG. 5 by means of lens stoppers 34, and the semiconductor laser 50 fixed by a laser holder 32 is installed in the lens barrel 30 on the left-band side of FIG. 5. The outer circumference of the laser holder 32 and the inner circumference of the lens barrel 30 have screw surfaces. By rotating the laser holder 32 so as to screw it into the lens barrel 30, the distance between the semiconductor laser 50 and the collimator lens 52 is reduced. By adjusting the distance between the semiconductor laser 50 and the collimator lens 52 in this way, an adjustment can be made so as to cause the slit beam to converge on the surface 60A to be measured which is at a predetermined position.
The optical axis of the aforesaid light receiving device is fixed at a predetermined angle 0 with respect to the optical axis of the light source 10. When the shape of the step formed in the surface to be measured 60A in the optical axis direction of the light source 10, the positions of the light spot on the surface 60A are displaced in the optical axis direction of the source 10 depending on the shape of the step, and the light beam impinging on the photodetector 58 is modified as shown in FIG. 4A. If a two-dimensional CCD sensor is used as the photodetector 58, the signals output by the photodetector 58 from for example two arbitrary lines 59A, 59B in the horizontal plane of the paper in FIG. 4A are as shown in FIG. 4B and FIG. 4C. The two-dimensional CCD sensor outputs these signals to the operating circuit 62.
Based on these input signals, the operating circuit 62 computes the position of the slit image 64 as a whole on the photodetector 58 by computing the positions of points of intersection (light points) between all the lines and slit image 64 on the photodetector 58. The shifts of these intersection points are calculated in order to define the shape of the step on the surface 60A. The shape of an object to be measured 60 can then be determined by computing this shape of the step over the whole of the photodetector 58.
As shown by the following equation (1), the aforesaid light point positions can be computed by the weighted average of the signals output from arbitrary lines on the photodetector 58. In other words, if intersection points are specified from a slit image on arbitrary lines, an average is found by weighting with the light amount irradiating positions on the photodetector (FIG. 8A). From this relation, the position of a light point can be found even if the light point on the photodetector 58 has reached a predetermined size: EQU Za={.SIGMA.(Ii.multidot.Zi)}/.SIGMA.Ii (1)
where
i=0, 1, . . . PA1 Za=position of light point PA1 Zi=position on photodetector PA1 Ii=amount of light irradiating position
This computation may also be performed by taking a simple average of positions irradiated on the photodetector by the laser beam at which the light intensity is above a preset threshold value Io. In other words, as shown in FIG. 8B, the intersection points Q1, Q2 where the output signal has the threshold intensity Io are computed, and positions Zb, Zc on the photodetector corresponding to the intersection points Q1, Q2 are then computed. A center value Zd of these computed positions Zb, Zc is defined as the light point position of the intersection between the slit image 64 and a line on the photodetector.
If a predetermined position on the photodetector 58 corresponding to a standard surface of the object to be measured 60 is first defined as a reference position, the displacement from the reference position of the surface to be measured 60A is expressed as a displacement of the image point on the photodetector 58. The displacement of the surface to be measured from the reference surface of the object 60 can then be computed by computing the shift of the computed slit image 64 with respect to the preset reference light point position based on the signal output according to the light image position on the photodetector 58.
However, it is known that the vertical/horizontal ratio of the light emitting part 50A of the semiconductor laser 50 is large. Depending on the direction of the light emitting part 50A when the semiconductor laser 50 is attached to the light source device, the width of the slit beam on the surface to be measured 60A is different as shown in FIGS. 7B and 7C. As the width direction of the slit beam is the measurement direction, if the width of the slit beam increases, the irradiation area required to specify the position of the surface to be measured increases. Due to the increase of irradiation area, the slit beam is more easily affected by such factors as surface roughness of the surface to be measured, and the sharpness of the light reflected by the surface 60A which irradiates the photodetector 58 decreases. As a result, the resolution required to detect the position decreases, and the position on the photodetector 58 corresponding to the value computed by the weighted average or simple average as described hereintofore, is displaced from the position of the real line so that a correct measured value cannot be obtained.
It is moreover known in the art that in the case of the semiconductor laser 50, the emergence (spread) angle of the laser beam is different in a direction aligned with the pn junction of the light emitting part 50A and in a direction traverse to the pn junction (FIG. 7A). For example, whereas the angle in the direction aligned with the junction of the semiconductor laser 50 is approximately 10.degree., the angle in the direction perpendicular to this junction is 30.degree. to 40.degree.. The divergence of the diffusing lens 52 therefore differs according to the direction of the light emitting part 50A when the semiconductor laser 50 is fitted to the light source device, and the light intensity near the edge of the slit beam irradiating the surface 60A varies. This relationship is shown in FIG. 6. In FIG. 6, Y corresponds to the position of the surface 60A, the intensity being a maximum at the optical center (0). If the light diverges in a direction aligned with the pn junction of the light emitting part 50A, therefore, the light intensity near the edge of the slit also falls, and the SN ratio of the output signal of the slit image corresponding to the edge of the slit beam irradiating the photodetector 58 decreases to a minimum.
Further, the output of the semiconductor laser 50 used in a conventional shape measuring device 12 is a low output of a few mW, and if it is desired to measure a surface 60A having a low reflectance, the amount of light received by the photodetector 58 falls. The SN ratio of the output signal from the photodetector 58 therefore deteriorates, and measurement errors occur. As the surface 60A of the object to be measured 60 will not necessarily have a high reflectance, a method was desired for measuring the surface shape of the object when its surface reflectance is low.
In order to measure the surface shape of the object 60 when the surface reflectance is low, a high power semiconductor laser 50 could possibly be used. In recent years, for example, as a result of technological advances, various semiconductor lasers having an output of 100 mW or more have been developed. Using these high power semiconductor lasers, the output signal from the photodetector 58 can be increased, and the surface shape of the object 60 having a low surface reflectance can be measured.
However, the vertical/horizontal ratio of the light emitting part is greater in the ease of a high power semiconductor laser than in the ease of a low power semiconductor laser. For example, the interval of the pn junction forming one side of the light emitting part is effectively the same, i.e. approximately 0.1 .mu.m, but the length of the direction aligned with the pn junction forming the other side is approximately 50 .mu.m in the ease of a high power semiconductor laser of 100 mW or more as compared to approximately 5 .mu.m in the case of a low power semiconductor laser. When the power of the semiconductor laser 50 increases, therefore, the surface area of the light emitting part increases. Even if the same optical system is used in the high power laser as that of the conventional low power laser, and the laser is rotated to attach it to the lens barrel 30, the center value or weighted value computed by the weighted average or simple average as described hereinbefore is even further displaced from the real position computed with a low power laser when the width of the slit beam irradiating the surface to be measured 60A increases depending on the direction of the light emitting part 50A.